1. Field of Invention
The present invention relates to a light emitting device driver chip; particularly, it relates to such light emitting device driver chip which is not directly connected to a rectified input voltage.
2. Description of Related Art
FIG. 1A shows a schematic view of a conventional light emitting diode (LED) driver chip 10 and the related circuits thereof. As shown in FIG. 1A, the LED driver chip 10 comprises pins P0-P4, a switch circuit 11, a switch control circuit 12 and a constant current source 13. The LED driver chip 10 drives an LED circuit 20. The LED circuit 20 comprises plural LEDs connected in series, e.g., LED1, LED2, LED3 and LED4 as shown in FIG. 1A. The switch circuit 11 includes four corresponding switches S1, S2, S3 and S4, which are electrically connected to the corresponding LEDs LED1, LED2, LED3 and LED4. An alternating current (AC) power source 40 generates an AC voltage. A rectifier circuit 30 rectifies the AC voltage to generate a rectified input voltage Vin as shown in FIG. 1B. The LED driver chip 10 drives the LED circuit 20 according to the level of the rectified input voltage Vin, to turn ON or OFF selected one(s) of the switches S1-S4, so that one or more of the LEDs LED1-LED4 of the LED circuit 20 emit light.
For example, as shown by the wave forms in FIG. 1B, when the level of the rectified input voltage Vin is lower than a level L1, none of the switches S1-S4 are turned ON, and none of the LEDs LED1-LED4 emits light. When the level of the rectified input voltage Vin is between the level L1 and a level L2, the switch S1 is turned ON whereas the switches S2-S4 are OFF, so that only LED LED1 emits light. When the level of the rectified input voltage Vin is between the level L2 and a level L3, the switch S2 is turned ON whereas the switches S1 and S3-S4 are OFF, so that LEDs LED1 and LED2 emit light. When the level of the rectified input voltage Vin is between the level L3 and a level L4, the switch S3 is turned ON whereas the switches S1-S2 and S4 are OFF, so that LEDs LED1-LED3 emit light. When the level of the rectified input voltage Vin is greater than the level L4, the switch S4 is turned ON whereas the switches S1-S3 are OFF, so that all LEDs LED1-LED4 emit light. The following patents contain relevant details as to how the LED driver chip 10 drives the LED circuit 20 according to the level of the rectified input voltage Vin: U.S. Pat. No. 6,989,807, U.S. Pat. No. 7,081,722 and U.S. Patent Publication No. 2011/0273102.
The constant current source 13 provides a current having a constant level, so that when one or more of the LEDs LED1-LED4 are emitting light, the current flowing through the one or more of the LEDs LED1-LED4 is a constant. As shown by the wave form of the current I1 in FIG. 1B, when at least one of the LEDs LED1-LED4 is emitting light, the current I1 is a constant regardless how many LEDs are emitting light. The level of the current I1 is equal to zero only when the level of the rectified input voltage Vin is lower than the level L1 (i.e., when none of the LEDs LED1-LED4 can emit light).
As compared to another type of LED driver which drives the LED circuit via a direct current (DC), the advantage of the conventional LED driver chip 10 is in that: it does not need to convert the rectified input voltage Vin to a DC voltage, thus reducing the manufacturing cost. If the frequency of the rectified input voltage Vin is sufficiently high, human eyes will not perceive the flicker of the LED circuit 20. However, one of the drawbacks of the conventional LED driver chip 10 is that: the operation voltage required by the switch control circuit 12 is also supplied from the rectified input voltage Vin generated by the rectifier circuit 30. As a result, for a LED driver chip 10 which is packaged into an integrated circuit (IC) chip, such LED driver chip 10 is required to further include an extra pin P0, in addition to the pins P1-P4 respectively connected to the LEDs LED1-LED4. That is to say, an additional pin P0 is required to connect to a node N0 for receiving the rectified input voltage Vin generated by the rectifier circuit 30, as a power source required for the operation of the LED driver chip 10. Consequently and undesirably, the pin P0 can not be omitted, thus hindering the IC chip from reducing its size and volume.
In view of the above, to overcome the drawback in the prior art, the present invention proposes a light emitting device driver chip which is not directly connected to a rectified input voltage. Therefore, the pin P0 required in the conventional LED driver chip 10 can be omitted, thereby reducing not only the size of the light emitting device driver chip but also the manufacturing cost.